Backport per-line-item tax adjustments.

I think when you originally created this for uc_vat, that's the way uc_vat used it. The hook was designed to return the tax (=taxable_amount * tax_rate) -- but uc_taxes collects the whole taxable amount and applies the rate at the end, so when i re-implemented for D7, I changed the hook to return the taxable amount. Can't make that change in D6 without breaking uc_vat integration, so we need divide the tax returned from the tax-adjustment callback by the rate to get the taxable amount.